a) Field of the Invention
This invention relates to a spark plug for an internal combustion engine, especially to the structure of a center electrode held on an insulator in a spark plug to be fitted in an internal combustion engine.
b) Description of the Related Art
Upon holding a center electrode in an insulator of a spark plug to be fitted in an internal combustion engine, it has heretofore been the practice to seal under heat the center electrode at a head portion thereof together with a terminal electrode with a glass sealing material within an axial bore of the insulator. It has been proposed in Japanese Patent Application Laid-Open (Kokai) No. HEI 2-165587 that a conical recess is formed in a head portion of a center electrode to increase the flowability of a molten glass sealing material upon heating and sealing the center electrode together with the glass sealing material within an axial bore of an insulator, thereby improving the bonding between the head portion of the center electrode and the glass seal and thus enhancing the fixing strength of the center electrode within the axial bore of the insulator, and also to facilitate header machining required for the head portion of the center electrode prior to the sealing.
The above-described conventional technique is however accompanied by drawbacks as will be described next. To fixedly hold the center electrode together with the terminal electrode within the axial bore of insulator, the glass sealing material is filled in the form of powder in the axial bore of the insulator and is then heated together with the center electrode to seal the center electrode. During this filling of the powdery glass sealing material and/or the heating and sealing of the center electrode, a pressure is exerted on the conical recess in the head of the center electrode, leading to a potential problem that the conical recess in the head portion of the center electrode may be deformed, specifically flattened. As a consequence, the mechanical fitting engagement between the head portion of the center electrode and the glass seal as intended to be achieved owing to the formation of the conical recess in the head portion of the center electrode, in other words, the bonding strength between the glass seal and the center electrode may be reduced. As the insulator is exposed to pressure of combustion gas produced as a result of combustion of an air-fuel mixture in a combustion chamber, the center electrode fixedly held within the axial bore of the insulator is required to have sufficient impact resistance. The center electrode may not be able to exhibit such sufficient impact resistance due to the above-mentioned reduced bonding strength.
Further, the above-mentioned deformation, namely, flattening of the conical recess in the head portion of the center electrode leads to a diametrical enlargement of the head portion of the center electrode. Although the glass sealing material is supposed to flow in the powdery form or in a molten form to the peripheral side wall of the flange portion of the center electrode upon filling the glass sealing material in the powder form or heating the same, the flow of the glass sealing material is blocked by the enlarged head portion of the center electrode. As a result, a space is formed along the peripheral side wall of the flange portion of the center electrode. It is therefore possible to produce only very low bonding strength between the insulator and the center electrode, leading to the problem that the sufficient impact resistance required for the center electrode cannot be achieved as in the above-mentioned case.